Tyrosyl peptides are being isolated from the binding sites of rabbit anti-hapten antibodies and of ligand-binding mouse myeloma proteins and their positions in the primary sequences of the heavy and light chains determined. The findings will show whether a combining- site tyrosyl residue is present at the same sequence position (and perhaps in a common tyrosyl sequence) in antibodies of a particular specificity in a given species and whether or not a common combining- site sequence may occur in more than one V-region sequence subgroup. They will also aid in establishing the role of the hypervariable sequence regions of the heavy and light chains in the combining site. Whether some binding sites contain two or more tyrosyl residues and whether they are from two different regions of the same heavy or light chain or one is from a light chain and one a heavy chain of the same molecule will be determined. The labeling of tyrosyl residues in the combining sites of the antibodies or ligand-binding myeloma protein is through the paired-iodination procedure. The significance of this work is that since there appears to be a limit to the number of different types of antibody molecules which can be formed in an individual, it seems that some sort of a pattern should exist for sequences forming particular antibody sites. If such patterns do exist and can be worked out, it will greatly facilitate the synthetic production of therapeutically active antibodies such as cytotoxic anti- cancer antibodies.